Interleukin-2 (IL-2), also known as T cell growth factor (TCGF), is a 15.5 kDa globular glycoprotein playing a central role in lymphocyte generation, survival and homeostasis. It has a length of 133 amino acids and consists of four antiparallel, amphiphatic α-helices that form a quaternary structure indispensable of its function (Smith, Science 240, 1169-76 (1988); Bazan, Science 257, 410-413 (1992)). Sequences of IL-2 from different species are found under NCBI RefSeq Nos. NP000577 (human), NP032392 (mouse), NP446288 (rat) or NP517425 (chimpanzee).
IL-2 mediates its action by binding to IL-2 receptors (IL-2R), which consist of up to three individual subunits, the different association of which can produce receptor forms that differ in their affinity to IL-2. Association of the α (CD25), β (CD122), and γ (γc, CD132) subunits results in a trimeric, high-affinity receptor for IL-2. Dimeric IL-2 receptor consisting of the β and γ subunits is termed intermediate-affinity IL-2R. The α subunit forms the monomeric low affinity IL-2 receptor. Although the dimeric intermediate-affinity IL-2 receptor binds IL-2 with approximately 100-fold lower affinity than the trimeric high-affinity receptor, both the dimeric and the trimeric IL-2 receptor variants are able to transmit signal upon IL-2 binding (Minami et al., Annu Rev Immunol 11, 245-268 (1993)). Hence, the α-subunit, CD25, is not essential for IL-2 signalling. It confers high-affinity binding to its receptor, whereas the β subunit, CD122, and the γ-subunit are crucial for signal transduction (Krieg et al., Proc Natl Acad Sci 107, 11906-11 (2010)). Trimeric IL-2 receptors including CD25 are expressed by (resting) CD4+ forkhead box P3 (FoxP3)+ regulatory T (Treg) cells. They are also transiently induced on conventional activated T cells, whereas in the resting state these cells express only dimeric IL-2 receptors. Treg cells consistently express the highest level of CD25 in vivo (Fontenot et al., Nature Immunol 6, 1142-51 (2005)).
IL-2 is synthesized mainly by activated T-cells, in particular CD4+ helper T cells. It stimulates the proliferation and differentiation of T cells, induces the generation of cytotoxic T lymphocytes (CTLs) and the differentiation of peripheral blood lymphocytes to cytotoxic cells and lymphokine-activated killer (LAK) cells, promotes cytokine and cytolytic molecule expression by T cells, facilitates the proliferation and differentiation of B-cells and the synthesis of immunoglobulin by B-cells, and stimulates the generation, proliferation and activation of natural killer (NK) cells (reviewed e.g. in Waldmann, Nat Rev Immunol 6, 595-601 (2009); Olejniczak and Kasprzak, Med Sci Monit 14, RA179-89 (2008); Malek, Annu Rev Immunol 26, 453-79 (2008)).
Its ability to expand lymphocyte populations in vivo and to increase the effector functions of these cells confers antitumor effects to IL-2, making IL-2 immunotherapy an attractive treatment option for certain metastatic cancers. Consequently, high-dose IL-2 treatment has been approved for use in patients with metastatic renal-cell carcinoma and malignant melanoma.
However, IL-2 has a dual function in the immune response in that it not only mediates expansion and activity of effector cells, but also is crucially involved in maintaining peripheral immune tolerance.
A major mechanism underlying peripheral self-tolerance is IL-2 induced activation-induced cell death (AICD) in T cells. AICD is a process by which fully activated T cells undergo programmed cell death through engagement of cell surface-expressed death receptors such as CD95 (also known as Fas) or the TNF receptor. When antigen-activated T cells expressing a high-affinity IL-2 receptor (after previous exposure to IL-2) during proliferation are re-stimulated with antigen via the T cell receptor (TCR)/CD3 complex, the expression of Fas ligand (FasL) and/or tumor necrosis factor (TNF) is induced, making the cells susceptible for Fas-mediated apoptosis. This process is IL-2 dependent (Lenardo, Nature 353, 858-61 (1991)) and mediated via STAT5. By the process of AICD in T lymphocytes tolerance can not only be established to self-antigens, but also to persistent antigens that are clearly not part of the host's makeup, such as tumor antigens.
Moreover, IL-2 is also involved in the maintenance of peripheral CD4+ CD25+ regulatory T (Treg) cells (Fontenot et al., Nature Immunol 6, 1142-51 (2005); D'Cruz and Klein, Nature Immunol 6, 1152-59 (2005); Maloy and Powrie, Nature Immunol 6, 1171-72 (2005), which are also known as suppressor T cells. They suppress effector T cells from destroying their (self-)target, either through cell-cell contact by inhibiting T cell help and activation, or through release of immunosuppressive cytokines such as IL-10 or TGF-β. Depletion of Treg cells was shown to enhance IL-2 induced anti-tumor immunity (Imai et al., Cancer Sci 98, 416-23 (2007)).
Therefore, IL-2 is not optimal for inhibiting tumor growth, because in the presence of IL-2 either the CTLs generated might recognize the tumor as self and undergo AICD or the immune response might be inhibited by IL-2 dependent Treg cells.
A further concern in relation to IL-2 immunotherapy are the side effects produced by recombinant human IL-2 treatment. Patients receiving high-dose IL-2 treatment frequently experience severe cardiovascular, pulmonary, renal, hepatic, gastrointestinal, neurological, cutaneous, haematological and systemic adverse events, which require intensive monitoring and in-patient management. The majority of these side effects can be explained by the development of so-called vascular (or capillary) leak syndrome (VLS), a pathological increase in vascular permeability leading to fluid extravasation in multiple organs (causing e.g. pulmonary and cutaneous edema and liver cell damage) and intravascular fluid depletion (causing a drop in blood pressure and compensatory increase in heart rate). There is no treatment of VLS other than withdrawal of IL-2. Low-dose IL-2 regimens have been tested in patients to avoid VLS, however, at the expense of suboptimal therapeutic results. VLS was believed to be caused by the release of proinflammatory cytokines, such as tumor necrosis factor (TNF)-α from IL-2-activated NK cells, however it has recently been shown that IL-2-induced pulmonary edema resulted from direct binding of IL-2 to lung endothelial cells, which expressed low to intermediate levels of functional αβγ IL-2 receptors (Krieg et al., Proc Nat Acad Sci USA 107, 11906-11 (2010)).
Several approaches have been taken to overcome these problems associated with IL-2 immunotherapy. For example, it has been found that the combination of IL-2 with certain anti-IL-2 monoclonal antibodies enhances treatment effects of IL-2 in vivo (Kamimura et al., J Immunol 177, 306-14 (2006); Boyman et al., Science 311, 1924-27 (2006)). In an alternative approach, IL-2 has been mutated in various ways to reduce its toxicity and/or increase its efficacy. Hu et al. (Blood 101, 4853-4861 (2003), US Pat. Publ. No. 2003/0124678) have substituted the arginine residue in position 38 of IL-2 by tryptophan to eliminate IL-2's vasopermeability activity. Shanafelt et al. (Nature Biotechnol 18, 1197-1202 (2000)) have mutated asparagine 88 to arginine to enhance selectivity for T cells over NK cells. Heaton et al. (Cancer Res 53, 2597-602 (1993); U.S. Pat. No. 5,229,109) have introduced two mutations, Arg38Ala and Phe42Lys, to reduce the secretion of proinflammatory cytokines from NK cells. Gillies et al. (US Pat. Publ. No. 2007/0036752) have substituted three residues of IL-2 (Asp20Thr, Asn88Arg, and Gln126Asp) that contribute to affinity for the intermediate-affinity IL-2 receptor to reduce VLS. Gillies et al. (WO 2008/0034473) have also mutated the interface of IL-2 with CD25 by amino acid substitution Arg38Trp and Phe42Lys to reduce interaction with CD25 and activation of Treg cells for enhancing efficacy. To the same aim, Wittrup et al. (WO 2009/061853) have produced IL-2 mutants that have enhanced affinity to CD25, but do not activate the receptor, thus act as antagonists. The mutations introduced were aimed at disrupting the interaction with the β- and/or γ-subunit of the receptor.
However, none of the known IL-2 mutants was shown to overcome all of the above-mentioned problems associated with IL-2 immunotherapy, namely toxicity caused by the induction of VLS, tumor tolerance caused by the induction of AICD, and immunosuppression caused by activation of Treg cells. Thus there remains a need in the art to further enhance the therapeutic usefulness of IL-2 proteins.